The present invention relates to a guide drum assembly for use in helical scan type magnetic recording/reproducing apparatus and a manufacturing method thereof.
A helical scan type a magnetic recording/reproducing apparatus (i.e., video tape recorder) generally includes a guide drum assembly for signal recording/reproducing which comprises an upper cylinder adapted to be rotatable about its own axis and to have at its circumferential surface a plurality of magnetic heads and a lower cylinder arranged to be stationary with respect to a base of said apparatus and to have on its circumferential surface a lead portion for guiding a magnetic tape. The upper and lower cylinders are coaxially provided so that on recording and reproducing a magnetic tape is travelled therealong so as to be guided by the lead portion of the lower cylinder and to wrapped around the circumferential surface of the upper cylinder to be in contact relation thereto.
A serious problem arising in such a magnetic recording/reproducing apparatus is that the magnetic tape vibrates when the traveling tape comes into an entry-side of the guide drum assembly where the tape is struck by an entry-side head to contact therewith and further, as described in detail in the monthly magazine "Television Technique" (published on May 1987), when the traveling tape leaves an exit-side of the guide drum assembly where an exit-side head departs to lose contact with the tape, thereby causing a jitter to occur on reproduction. One possible solution for the tape vibration due to the separation from the exit-side head is to provide to the guide drum assembly a projection (so-called bulge), whose height is about 50 micrometers, so as to reduce a contact pressure due to a generated vacuum between the magnetic tape and the exit-side of the guide drum to allow smooth separation of the magnetic tape therefrom, as described in the above-mentioned monthly magazine. However, because of locational design limitations, difficulty is encountered to provide a similar projection for prevention of the tape vibration due to the striking contact with the entry-side head. Therefore, in order to prevent the magnetic tape from vibrating or coming into contact with the entry-side head, an arrangement of a lower cylinder, coaxially arranged with an upper cylinder 8, has been proposed as shown in FIG. 1, where a L-shaped member 5 having a projection 4 is fitted by means of a screw 6 in a cylinder body 3, the projection 4 protruding through an opening from the outer surface of the cylinder body 3 and the protruding amount thereof being adjusted by means of an adjusting screw 7 in a range between 5 to 10 micrometers from the outer surface. This arrangement requires a complex construction and an increase of the manufacturing cost.